Interference filter having a glass substrate

ABSTRACT

The present invention relates to a fiber optic system including a light source, a fiber optic transmission component, a receiver or transmitter of radiation, and an interference filter. The interference filter may include a glass substrate with at least two interference layers coated thereon. The glass substrate can include:  
                                           Oxide   Range                   P 2 O 5     30-70         Al 2 O 3      5-15         R′O    5-15         R′ = Mg, Ca, Sr, Ba, Zn, Pb         R 2 O R = Li, Na, K   15-40                                    
 
     where the glass substrate has a coefficient of, thermal expression of 130−210×10 −7 /° C. at 30° C. to 70° C.

FIELD OF THE INVENTION

[0001] This invention generally relates to an interference filter having a glass substrate.

BACKGROUND OF THE INVENTION

[0002] Substrates that demand high expansion with good chemical durability are often manufactured from optical glasses. Optical glasses may be employed in various applications, such as substrates for interference filters used in fiber optic systems. Generally, these interference filters are fabricated from multiple layers of conducting and insulating materials or films that together result in a filter that passes only a narrow bandwidth of incident radiation. Such filters are described in Optical Filter Design and Analysis-A Signal Processing Approach by Christie K. Madsen and Jian H. Zhao published by John Wiley & Sons, 1999.

[0003] The thermal expansion value of a glass substrate can have important implications on device performance. As an example, a mismatch in thermal expansion between the glass substrate and a coating film can impose undue stress on the film. This stress can be calculated by the following formula:

σ=E _(film) ΔαT

[0004] where E_(film) is the Young's modulus of the film, At is the mismatch in thermal expansion coefficient between the film and substrate, and AT is the shift in temperature from the preparation temperature of the film to the temperature of use, which is usually room temperature.

[0005] One solution is to prepare and maintain the film at the temperature of its intended use. However, transient stresses develop even for slight departures from the film creation temperature. Therefore, it is highly desirable to minimize the mismatch in thermal expansion between the film and the glass substrate.

[0006] In one particular application, there is a strong demand for a glass substrate capable of being incorporated into an interference filter for dense wave division (DWD) or dense wave division multiplexing (DWDM) applications. Such interference filters have high requirements in narrowing the bandwidth of light transmission with minimal drift of this property with change in temperature. These filters require bandwidths of less than 200 GHz pass frequency in the 1.5 μm wavelength region. Desirably, the substrate should be characterized by high transmission in the near IR where DWDM systems operate, i.e., wavelengths at or near 1.5 μm, and a refractive index value at 587.6 nm, (n_(d)) of 1.50 to 1.60. High transmission at 1.5 μm is characterized by a value of digital transmittance exceeding 88%.

[0007] It is also desired that such substrate glasses not contain primary colorants such as Mn, Co, Ni, V, Fe, Cr, and Cu, e.g., because they can impede transmission or not contain rare earth additives such as Er oxide or Nd oxide, which can have absorption bands in the visible or near-infrared regions of the electromagnetic spectrum, or not include CoO, NiO₂, FeO, Fe₂O₃ and CuO which can impede transmission in the near infrared region of the electromagnetic spectrum.

[0008] Furthermore, it would be desirable to manufacture an interference filter substrate economically. One such desirable process for making interference filter substrates from optical glasses is tank melting. In this case, refining agents such as arsenic oxide and antimony oxide are often used during the tank melting process to produce glass with high optical quality and low bubble content. However, often optical glass substrates contain cerium oxide. Cerium can form a solarization couple with either of these compounds, resulting in browning of the glass when it is exposed to short wavelength radiation. The glass browning can cause the loss in optical transmission. Consequently, glasses having cerium oxide would generally be undesirable for use as an interference filter substrate.

SUMMARY OF THE INVENTION

[0009] A desired embodiment of the invention is an interference filter for a fiber optic system including a substrate and a film coating the substrate. Typically, the substrate is coated with a series of layers of differing materials having properties, e.g., indices of refraction, producing interference effects achieving the desired wavelength transmission spectrum. Fiber optic systems comprise one or more light sources, fiber optic transmission components, a receiver of transmitted radiation, filters and end use components, e.g., detection, amplifiers, etc. In the prior art, it was desirable to have a glass substrate match the thermal expansion of a coating film. Surprisingly, it has been found that glass substrates having a thermal expansion much higher than that of the coatings applied to the substrate have significant and unexpected properties as a filter, such as having transmission characteristics independent of temperature. Generally, substrates having a coefficient of thermal expansion of about 130—about 210×10⁻⁷/° C. (−30° C. to +70° C.) are useful in this invention.

[0010] One exemplary embodiment of the present invention relates to a fiber optic system. The fiber optic system can include a light source, a fiber optic transmission component, a receiver or transmitter radiation, and an interference filter. The interference filter can include a glass substrate with at least two interference layers coated thereon. The glass substrate may include: Oxide Range P₂O₅ 30-70 Al₂O₃  5-15 R′O  5-15 R′ = Mg, Ca, Sr, Ba, Zn, Pb R₂O R = Li, Na, K 15-40

[0011] and have the following properties: Measurement Range nd 1.50-1.60 Digital Transmittance at 1.5 μm >88% CTE 130-210 (−30 to +70° C.)

[0012] Another exemplary embodiment of the present invention relates to an interference filter having a glass substrate. The glass substrate may have at least two interference layers coated thereon. What is more, the interference filter glass substrate can include: Oxide Range P₂O₅ 30-70 Al₂O₃  5-15 R′O  5-15 R′ = Mg, Ca, Sr, Ba, Zn, Pb R₂O R = Li, Na, K 15-40

[0013] and have the following properties: Measurement Range nd 1.50-1.60 Digital Transmittance at 1.5 μm >88% CTE 130-210 (−30 to +70° C.)

[0014] A still further exemplary embodiment of the present invention is a glass substrate. A glass substrate can include: Oxide Range P₂O₅ 35-60 Al₂O₃ 10-14 R₂O R = Li, Na, K 30-35 Li₂O 0-3 Na₂O  0-20 K₂O  0-20 R′O  5-10 R′ = Mg, Ca, Sr, Ba, Zn, Pb La₂O₃ and/or B₂O₃ 0-5

[0015] Furthermore, the glass substrate can have the following properties: Measurement Range nd 1.50-1.60 Digital Transmittance At 1.5 μm >88% CTE 130-210 (−30 to +70° C.)

[0016] An additional exemplary embodiment of the present invention relates to a method of making a fiber optic system. The method can include providing an interference filter having a glass substrate including: Oxide Range P₂O₅ 30-70 Al₂O₃  5-15 R′O  5-15 R′ = Mg, Ca, Sr, Ba, Zn, Pb R₂O R = Li, Na, K 15-40

[0017] Still another exemplary embodiment of the present invention relates to a method of passing light through an interference filter. The method can include passing light through an interference filter having a glass substrate including: Oxide Range P₂O₅ 30-70 Al₂O₃  5-15 R′O  5-15 R′ = Mg, Ca, Sr, Ba, Zn, Pb R₂O R = Li, Na, K 15-40

[0018] What is more, the passed light can be transmitted through a fiber optic transmission component.

[0019] Several properties are important for the substrates of such filters, especially interference filter components. These properties include, especially, coefficient of thermal expansion and refractive index at 587.6 nm, (nd), and high digital transmittance at 1.5 μm. As depicted below in Table 1, glass substrates of the present invention desirably have the following properties in order to provide useful substrates for interference filters: TABLE 1 Measurement General Range Preferred Range Optimal Range n_(d) (587.6 nm) 1.50-1.60 1.52-1.58 1.54-1.56 Digital >88% >89% >90% Transmittance at 1.5 μm CTE 130-210 135-200 140-180 (−30 to +70° C.)

[0020] Glasses which achieve these properties include (in mol %): TABLE 2 Mol % General Preferred Oxide Range Range Optimal Range P₂O₅ 30-70  35-65  35-60 Al₂O₃ 5-15 7-14 10-14 R₂O (R = Li, Na, K) 15-40  25-35  30-35 Li₂O 0-20 0-12 0-3 Na₂O 0-30 0-25  0-20 K₂O 0-30 0-25  0-20 R′O 5-15 5-12  5-10 R′ = Mg, Ca, Sr, Ba, Zn, Pb La₂O₃ and/or B₂O₃ 0-10 0-8  0-5

[0021] These glasses are essentially free of Fe₂O₃ and other additives that would restrict transmission at telecommunicating wavelengths, i.e., in the near IR, at or near 1.5 μm.

[0022] Optionally, these glasses also include: Mol % General Preferred Optimal Oxide Range Range Range ZnO 0-10 0-8 0-5 TiO₂ 0-10 0-8 0-5 ZrO₂ 0-10 0-8 0-5 MoO₃ 0-10 0-8 0-5 Ta₂O₅ 0-10 0-8 0-5 WO₃ 0-10 0-8 0-5 In₂O₃ 0-10 0-8 0-5 Y₂O₃ 0-8 0-5 0-5

[0023] These P₂0₅ substrates of the present invention maybe made by conventional glass melting techniques. Raw materials can be melted in platinum crucibles and aired at temperatures around 1200° C. for up to five hours.

[0024] The interference filter for a fiber optic system also includes at least one film desirably in the form of a layer. Such films can be selected from SiO₂, Ta₂O₅, HfO₂, etc. These can be applied by commercially available standard ion beam deposition systems such as the SPECTOR™ system available from Ion Tech, Inc. of Fort Collins, Colo., and other known methods. In addition to being particularly useful for DWDM filters, these glasses are also exceptionally useful as high expansion glasses for fabrication of hybrid structures that demand a high expansion glass with good chemical durability, e.g., for the purposes of longwave pass filters, polarizing components, band pass filters, etc.

[0025] Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

[0026] In the foregoing and in the following example, all temperatures are set forth uncorrected in degrees Celsius; and, unless otherwise indicated, all parts and percentages are by mole based on oxide.

[0027] The entire disclosures of all applications, patents and publications, cited above or below are hereby incorporated by reference.

[0028] An exemplary glass substrate prepared by methods of the present invention is presented in Table 3 below: TABLE 3 Oxide Mol % P₂O₅ 40 Al₂O₃ 12 Na₂O 15 K₂O 18 PbO  9 B₂O₃  6 Property Value n_(d) (587.6 nm) 1.549 Digital Transmittance 91% at 1.5 μm CTE 150.4 × 10⁻⁷/C (−30 to +70° C.)

[0029] Thermal expansion coefficients of the glass compositions, determined by dilatometry were calculated from the total glass expansion measured from room temperature just below the glass transition temperature.

[0030] The preceding example can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding example.

[0031] From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 

1. A fiber optic system, comprising: a light source; a fiber optic transmission component; a receiver or transmitter of radiation; and an interference filter, comprising a glass substrate with at least two interference layers coated thereon; wherein the glass substrate comprises: Oxide Weight Percent Based on Oxide P₂O₅ 30-70 Al₂O₃  5-15 R′O  5-15 R′ = Mg, Ca, Sr, Ba, Zn, Pb R₂O R = Li, Na, K 15-40

and wherein the glass substrate has a coefficient of thermal expansion of 130-210×10⁻⁷/° C. at 30° C. to 70° C.
 2. A fiber optic system, comprising: a light source; a fiber optic transmission component; a receiver or transmitter of radiation; and an interference filter, comprising a glass substrate with at least two interference layers coated thereon; wherein the glass substrate is made by adding together and melting: Oxide Weight Percent P₂O₅ 30-70 Al₂O₃  5-15 R′O  5-15 R′ = Mg, Ca, Sr, Ba, Zn, Pb R₂O R = Li, Na, K 15-40

and wherein the glass substrate has a coefficient of thermal expansion of 130−210×10⁻⁷/° C. at 30° C. to 70° C. 